The present invention relates to a device and a method for manufacturing paper used in a thermal printer, and to particulate coating composite coated on the paper for forming an image forming layer.
The spread of personal computers and office computers has caused the wide use of printers capable of printing an image drawn with the computer in color. Thermal printers are usually used for printing out such a color image.
The thermal printer melts ink disposed on an ink sheet with a head thereof, and transfers the ink to a sheet of image forming paper, exclusively provided for thermal printers. The thermal printer roughly uses either of the two methods, namely, dye melting transfer system, and dye sublimating transfer system. In the dye melting transfer system, the ink sheet is heated and the melted ink is transferred to paper. Although the heat applied to each dot on the pint head makes it difficult to achieve a high resolution of the printed image, due to the technological improvement for miniaturization in recent years, such as the use of a head made of a single crystal silicon, the heat stabilization is improved, thereby easily controlling the heat of the head. Hence, a printer capable of printing a high resolution image is known.
On the other hand, the thermal printer using the dye sublimating transfer system heats an ink sheet having a sublimating dye as a main component. The vaporized dye is reacted with polyester resin which is formed on paper as the image forming layer, thereby transferring the image. By adjusting the heating quantity, the tone of each dot can be arbitrarily controlled. To the contrary, in the dye melting transfer system, the size and the darkness of the printed dot is constant.
The ink sheet for the dye melting transfer system comprises a coated film, for example, of polyethlene terephtalate formed on a carrier. On the other hand, the paper used in the dye sublimating transfer system is composed as follows. On a substrate comprising ordinary paper, synthetic paper or a sheet of synthetic resin, there is formed an image forming layer comprising a resin layer on which dye can be diffused or migrated, and a mold-releasing layer for preventing the sticking of the image forming layer on the ink sheet. The image forming layer has a coating composite of various resins including saturated polyester resin, and denatured silicone oil dissolved in organic solvent which is applied on the substrate such as synthetic paper and dried. The mold-releasing layer is formed by heating the image forming layer at a high temperature of more than 100.degree. C., thereby allowing the silicone oil to bleed over the surface of the image forming layer, and thereafter cured.
The coating composite of the image forming layer used to be liquid. However, the use of a particulate composite has been considered and actually put into practice. Moreover, there has been known a technique of rendering the manufactured paper white as the ordinary paper by using a white particulate coating composite. The paper employing the particulate coating composite during the manufacture thereof can be used both for the dye sublimating transfer system and the dye melting transfer system.
FIG. 2 shows a device for manufacturing such a conventional paper. The device comprises a substrate feeder section 1, coating composite supply section 2, coating composite apply section 3, fixing section 4, and a discharge section 5. The substrate feeder section 1 comprises a pair of rollers and a carrier means. A paper substrate 7 (FIG. 3) on which the image forming layer is not yet formed, is delivered to the coating composite apply section 3 through the pair of rollers and the carrier means. The carrier means may be a well known device such as rollers and belts.
As shown in FIG. 3, the coating composite supply section 2 has an coating composite storage 8. Although not shown in the figure, the coating composite supply section 2 is further provided with a supply sleeve, blade, and a latent image forming device. In the case where paper is manufactured using a particulate coating composite as the coating composite, particulate coating composite 9 is stored in the coating composite storage 8.
The use of the particulate coating composite 9 is advantageous in that the manufacturing process can be simplified and the cost thereof is reduced. Namely, in the method where solution is applied on the substrate, various processes such as application, drying, and forming a mold-releasing layer are necessary. On the other hand, the process can be simplified when the particulate coating composite is used, thereby enabling to reduce the manufacturing cost.
In the device shown in the figures, the particulate coating composite 9 is white so that the manufactured paper is white as the ordinary paper. The particulate coating composite 9 is fed to the supply sleeve and the blade adjusts the thickness of the layer of the particulate coating composite 9 formed on the sleeve. The latent image forming device is provided to migrate the white particulate coating composite 9 from the developing sleeve to the substrate, and in the example shown in FIG. 3, comprises a charged drum 6 made of a photosensitive material. When a laser beam is radiated on the surface of the charged drum 6, a latent image represented by dots is formed.
As disclosed in Japanese Patent Application Laid Open 8-224970, The white particulate coating composite includes thermoplastic resin, white pigment as a colorant, and various waxes as an offset preventing agent. The offset preventing agent is included so as to improve the releasing characteristic between the thermoplastic resin and a roller which is provided in the fixing section 4. Namely, the melted offset preventing agent spread over the interface between the roller and the resin improves the releasing characteristic. Hence, the resin which ought to adhere on the substrate is prevented from adhering to the roller. The composition of the white particulate coating composite is explained in the above mentioned publication, so that further detailed description thereof is omitted.
As the charged drum 6 of the coating composite supply section 2 rotates, the white particulate coating composite 9 adhered on the surface thereof is carried to the transfer section. In the transfer section, a charge of opposite polarity to that of the white particulate coating composite 9 is applied to the substrate 7 at the underside thereof in FIG. 3, that is the opposite side of the substrate 7 to the side where the particulate coating composite is to be adhered. Hence, due to the electrostatic force, the white particulate coating composite is transferred to the substrate 7 in the form of dots. In the coating composite apply section 3, the white particulate coating composite 9 still adhering on the charged drum 6 at the transfer operation is removed by a well known cleaner section. The substrate feeder section 1, coating composite supply section 2 and the coating composite apply section 3 compose an application means in the description of the present invention.
The substrate 7 on which the white particulate coating composite 9 is adhered in the coating composite apply section 3 is fed to the fixing section 4 by a carrier means. As shown in FIGS. 4 and 5, the fixing section 4 comprises a fixing roller device 12 comprising a pair of rollers 10 and 11 for pressing and heating the substrate 7. When the substrate passes through a gap formed between the rollers 10 and 11, the white particulate coating composite 9 on the substrate 7 is pressed and heated so as to be melted, thereby forming an image forming layer 13 on the substrate 7. The substrate is then discharged from the manufacturing device through the discharge section 5.
In the fixing section, the offset preventing agent included in the white particulate coating composite is also melted when the substrate 7 is heated. Hence, the offset preventing agent is spread at the interface between the roller 10 and the resin in the particulate coating composite, thereby improving the releasing characteristic. As a result, the resin is prevented from adhering on the roller 10. Accordingly, the heat applied by the rollers is so controlled that the white particulate coating composite 9 including the offset preventing agent is melted.
Although there are other fixing means besides the one shown in FIGS. 4 and 5, the device shown in the figures, known as a heat roll fixing type employing heat and pressure, is most commonly used today. This is because the device provides good heat efficiency, improved safety, and stable fixing.
Referring to FIG. 5, describing the device more in detail, a heater 14 is provided inside the upper roller 10 so as to heat the composite 9 and the substrate 7. A halogen lamp may be used as the heater 14. The lower roller is urged by an elastic member (not shown) toward the upper roller, thereby pressing the substrate 7. By thus heating and pressing the substrate 7, the white particulate coating composite 9 is adhered on the substrate. In addition, a thermistor for controlling the fixing temperature, a thermoswitch 16, and a release pawl 17 for preventing the substrate 7 from winding on the roller 10 are provided.
The paper for the thermal printer is manufactured with such a device. In the last manufacturing process, the paper is cut into a sheet of a predetermined size. A predetermined number of sheets are packed and shipped. The paper is loaded in a thermal printer and a color image drawn with a computer is printed thereon.
In the thus manufactured paper, the offset preventing agent is melted with the resin comprising the white particulate coating composite. As explained above, the offset preventing agent is included so as to improve the releasing characteristic of the melted particulate coating composite from the roller 10. That is, since the melted offset preventing agent is spread in the area where the roller 10 contacts the white particulate coating composite, the releasing characteristic is improved. The offset preventing agent is heated by the heater described above, diffused across the surface of the substrate 7, and cooled after the heating, that is after passing between the rollers 10 and 11. Hence the offset preventing agent is spread over the entire surface of the substrate 7. However, such a completely spread offset preventing agent causes ink to be repelled when printing in the thermal printer, thereby forming blank portions in the image. This occurs in either types of printer employing the dye melting transfer system or the dye sublimating transfer system. Namely, in the dye melting transfer system, the bonding force between the ink and the image forming layer is weakened by the spread offset preventing agent. In the dye sublimating transfer system, the diffusion of the ink in the image forming layer is deteriorated by the offset preventing agent. Therefore, there is sought a technique for removing such a disadvantage without decreasing the releasing characteristic.